1. Field of the Invention
This invention relates to an aligning system such as, for example, one to be used in the printing of semiconductor circuits in which high precision is required in transferring an image from a photomask to a wafer.
2. Description of the Prior Art
In the field of semiconductor circuit manufacture, very precise exposure that produce high quality exposures are desired in order to produce very fine-patterned and highly integrated circuits. This is particularly so in view of the trend of developments from ICs (integrated circuits to LSIs (large-scaled integrated circuits) to VLSIs (very large-scaled integrated circuits). Important capabilities required for such exposure apparatus are as follows:
(1) Printing performance for permitting printing of fine patterns of the order of 1 to 2 .mu.m;
(2) Positioning precision for accurately positioning the pattern on a photomask relative to the pattern on a wafer which has been printed thereon from a photomask in the preceding printing step; and
(3) Transfer precision for projecting the image of the photomask onto the wafer without magnification and distortion errors.
FIG. 1 shows a known semiconductor printing apparatus in which a wafer 3 is slit-exposed with the pattern of a photomask 1 through a mirror imaging optical system. The photomask and wafer are moved as in unison in the direction (shown by the arrows) perpendicular to the length of the slit to complete the entire exposure. In such semiconductor printing apparatus, if the photomask and wafer are displaced in unison in the lateral direction (X) of the slit, in the rotational direction (.theta.) in a plane containing both the longitudinal direction of the slit and the above-mentioned lateral moving direction along which the photomask and wafer are moved, or in the direction (Y) in which the photomask and wafer are inclined with respect to the moving direction in a plane perpendicular to the longitudinal direction of the slit, during the aligning step prior to the exposure step, it is observed through an alignment optical system that the alignment marks on the photomask and wafer are displaced relative to each other by the amount two times the above displacement.
Therefore, the alignment between the alignment marks on the mask and wafer, i.e., the alignment between the mask and wafer through the aligning optical system is obtainable by displacing the mask and wafer as a unit in the direction X, Y or .theta..
As a mechanism for displacing the photomask and wafer, there is known a combination of movable stages utilizing rolling or sliding of balls with drives such as motors, cylinders or the like. Although in such mechanism, the amount of movement can be large, the movable stages may stick-and-slip and sometimes cannot move smoothly. Further, the drive system has a backlash which adversely affects the response in fine movement of the stages. Thus, the prior art mechanism for displacing the photomask and wafer requires a prolonged time period to complete the aligning step and can result in reduced accuracy and operability.
The above-mentioned problems themselves may be solved if a fluid bearing is used and the supply pressure to floating elements is controlled to ensure the alignment. With such means, however, the absolute amount of movement will not be large since the gap between the guide and floating element is limited due to the rigidity thereof .